1. Field
The present invention relates to a termination connection box for a direct-current (DC) cable, and more particularly, to a termination connection box capable of decreasing a resistance value at an end portion of a DC cable and forming a uniform resistance distribution to prevent an electric field from being locally concentrated.
2. Description of the Related Art
In general, power cables which are devices transferring power using a conductor therein may be classified into a direct-current (DC) power cable and an alternating-current (AC) power cable.
When a joint box or a termination connection box according to the related art is used to connect ends of DC power cables or connect a DC power cable to an aerial wire, the system is complicated in structure and thus it may take a large amount of time to install or build it.
To solve this problem, in the related art, when a termination connection box structure for a DC power cable is provided, a conductor of this cable is electrically connected to an outer semiconducting layer by applying resistive paint onto an insulator region of an end portion of this cable, from which the outer semiconducting layer is removed. Thus, a resistance component is distributed between the conductor and the outer semiconducting layer and thus an electric field may be divided.
However, in the termination connection box according to the related art, when the resistive paint is applied onto a surface of an insulator of the cable, the resistive paint should be applied to a predetermined thickness or more so as to obtain the effect of dividing an electric field. Thus, a process of repeatedly applying the resistive paint several times should be performed. When this process is performed, it is difficult to maintain the applied resistive paint to a uniform thickness and thus a uniform resistance distribution may not be achieved. Furthermore, a foreign substance such as moisture may penetrate an interface between the insulator and the resistive paint and the applied paint or a void may be formed at the interface between the insulator and the resistive paint and the applied paint.
U.S. Pat. No. 4,737,318 (hereinafter referred to as ‘Document 1’) discloses a joint box or a termination connection box for an AC cable, in which a first layer covering a portion of an insulating layer and formed of a heat shrinkable tube, an external heat shrinkable tube, and a second layer coated onto an inner side of the external heat shrinkable tube are provided. The first layer has a linear AC electrical property and the second layer has a non-linear AC electrical property. However, Document 1 relates to connection of an AC cable, in which the first layer and the second layer are not configured to connect a conductor to an outer semiconducting layer.
US 2013-0078836 (hereinafter referred to as ‘Document 2’), US 2013-0075124 (hereinafter referred to as ‘Document 3’), and EP 2639264 (hereinafter referred to as ‘Document 4’) disclose structures of a termination unit of a DC cable which includes a layer formed of a so-called field grading material (FGM). However, in Documents 2 and 3, an FGM layer is formed by coating or painting or in the form of a sleeve or tape. When the FGM layer is formed by coating or painting or in the form of tape, it is very difficult to form the FGM layer to a uniform thickness while preventing a foreign substance from penetrating the FGM layer. When the FGM layer is formed in the form of sleeve, the FGM layer may be formed to a uniform thickness but it is difficult to prevent a foreign substance from penetrating the FGM layer or a void from being generating in the FGM layer. In Document 4, an FGM layer includes an injection layer. However, when the FGM layer is an injection layer, it is not easy to install and assemble the DC cable.
PCT/EP2012/063041 (hereinafter referred to as ‘Document 5’) discloses a joint box for a high-voltage cable, in which a resistance layer having a region which gradually increases toward an external triple point is provided. The resistance layer changes the position of the triple point and thus the concentration of an electric field is reduced. However, in Document 5, the resistance layer simply changes the position of the triple point and thus the effect of concentrating an electric field is low. Furthermore, the resistance layer is formed by injection molding or the like and it is thus difficult to install and assemble the DC cable, similar to Document 4.